Strategies for Separating Overlapping Effects, Part 2: TGA - METTLER TOLEDO

Strategies for Separating Overlapping Effects, Part 2: TGA

The interpretation and evaluation of thermal analysis measurement curves is difficult when several effects take place simultaneously. A number of methods are available that can be used to separate overlapping effects and analyze them individually afterward. In this article, we discuss strategies for TGA curves using suitable examples.

 

Introduction

The interpretation and quantitative evaluation of thermal analysis measurement curves is difficult when several effects occur simultaneously. For TGA measurements, there are three main strategies that can be applied to separate overlapping effects:

Normally a frequency of 1 Hz is used and the measurement is often performed in the bending mode or (with films) in tension. However, since the introduction of modern DMA instruments like the METTLER TOLEDO DMA861e, much more information about the properties of materials can be obtained by varying the measurement conditions. The DMA861e is capable of measuring sample stiffness over a very large range. The possibilities now available are discussed using different polymers and composites as examples.

a) Variation of the temperature program. This includes using different heating rates, performing heating-cooling-heating experiments, and using "MaxRes" (a method in which the heating rate is automatically changed).

b) Changing the environment directly around the sample. This includes using different gases, different gas pressures, and different crucibles.

c) The use of TGA-EGA. If different decomposition products are formed when mass losses overlap, the substances involved can be identified and quantified with the aid of evolved gas analysis techniques such as TGA-MS, TGA-FTIR, TGA-GC/MS, and TGA-Micro GC/MS.

We will discuss these strategies in the following sections and illustrate them with suitable examples.

 

Variation of the Temperature Program

Variation of the heating rate

In general, reactions shift to higher temperatures at increasing heating rates. The extent to which this occurs is different for every reaction and also depends on the dimensions of the sample, temperature gradients in the sample, reaction kinetics or the diffusion of decomposition products out of the sample. This rate dependence can be used to separate overlapping reactions.

As an example, Figure 1 shows TGA curves of copper sulfate pentahydrate (CuSO4·5H2O) measured at heating rates of 5 K/min and 25 K/min. Three mass loss steps can be seen in both measurements. They correspond to the loss of two molecules of water in both the first and second steps and one molecule of water in the third step from the initial CuSO4·5H2O molecule.

The first derivatives of the TGA curves (DTG curves, small inset diagram) clearly show that the first two mass loss steps overlap at a heating rate of 25 K/min so that the corresponding mass losses cannot be properly quantified. If the same sample is measured at 5 K/min (red curve), the first two steps are clearly separated (the DTG curve reaches zero).

Conclusions

Different temperature programs (different heating rates, heating-cooling-heating cycles or MaxRes), the optimization of environmental parameters such as the gas, pressure and crucibles as well coupling the TGA to other techniques are all useful methods for separating overlapping effects on TGA curves.

 

Strategies for Separating Overlapping Effects, Part 2: TGA | Thermal Analysis Application No. UC 465 | Application published in METTLER TOLEDO Thermal Analysis UserCom 46